Uterine Abnormalities

Miscarriage can also be caused by an inherent abnormality in the structure of the uterus. There are two basic types of congenital structural abnormalities of the uterus, and there are several subvarieties. One type of congenital abnormality is a double, or bicornuate, uterus, which simply means that instead of one uterus there are two, usually with a common cervical opening (see figure below). This is very similar to the uterine structure of most animals that have two uterine horns. At six weeks of fetal life, the Müllerian ducts are paired structures that fuse and develop into the uterus, the upper vagina, and the fallopian tubes. In most animals, the Müllerian ducts do not fuse, and so these animals have a double uterus normally. In humans, they fuse into one uterus and one upper vagina, but the fallopian tubes remain separate. Bicornuate uterus is simply caused by the failure of the original Müllerian ducts, at six weeks of fetal life, to fuse in the center and form a single uterus. Thus, a woman who has a bicornuate uterus really has a normal variation that is closer to most of the animal kingdom than the usual human single uterus. This bicornuate uterus may look abnormal and you may be told by your doctor that it is a problem, but it really isn’t a problem.

The problem is with the so-called septate uterus, in which the two Müllerian ducts fuse partially but not completely (see figure below). In this case, there appear to be two uterine cavities, but they are enclosed within one common muscular uterine exterior. With this type of uterine abnormality, which is often confused with the bicornuate type, the uterus cannot expand the way it is supposed to in pregnancy, and there is definitely a higher incidence of miscarriage. This congenital problem of septate uterus can be corrected with simple hysteroscopic surgery. However, for the bicornuate uterus, nothing should be done because it simply means that you have two uteruses instead of one, and either one of those two uteruses can handle a pregnancy normally without an increased risk of miscarriage.

One of the most compelling cases of an absolutely absurd-looking congenitally abnormal uterus, which created lots of fears but in truth was not the source of the problem, was a patient who came to us in the early days of ICSI. Her husband’s sperm count was severely diminished, with less than half a million sperm per cc, and this was clearly the cause of her infertility. However, a hysterosalpingogram X-ray in her local community revealed what’s called a unicornuate uterus (which is like a bicornuate uterus except that there’s only one uterine horn instead of two). This is a very oddly misshapen form in which a single cervix leads into half of a uterus. The doctors in her area were certain she could never carry a baby with such a strange-looking uterus. Yet this woman delivered healthy twin girls after her ICSI procedure, and the fact that she had this funny-looking unicornuate uterus had no negative impact on her ability to get pregnant or to carry the twin pregnancy fully to term.

Another compelling example of this phenomenon is a woman in her fifties who came to us from Europe because she and her husband required donor eggs. Her HSG, which had been performed many years ago in Europe, revealed a bizarre-looking type of bicornuate uterus in which she actually had two cervical openings and two completely independent uterine structures. However, she was not concerned about this because her doctors in Europe were very sophisticated and had explained to her that this would not present a problem for carrying a pregnancy. When we performed her IVF with donor eggs, we transferred two embryos into one of her two uteruses and one embryo into the other uterus, rather than putting all three into one of them. As it turned out, she carried a twin pregnancy that developed from the embryos implanted into one uterine cavity, and that single uterine cavity was quite sufficient. She delivered normal, healthy twins nine months later despite this odd-looking and apparently abnormal bicornuate uterus.

A fifty-two-year-old woman had come to our clinic in St. Louis fifteen times and had attempted to get pregnant for the previous eight years with fifteen cycles of IVF. Her perseverance was amazing. Finally, after many years, she agreed to use donor eggs. All along she was noted to have several large fibroid tumors, each three inches in diameter, within the muscle of her uterus. Her doctors at home were concerned that these large fibroids in her uterus might interfere with pregnancy, but they were just in the wall of the uterus and did not protrude inside the uterine cavity. So we ignored them, and she got pregnant (using donor eggs) and delivered healthy twins. Thus, in most cases, the embryo is still more important than the uterus.

In summary, miscarriage is not always caused by chromosomal abnormalities in the embryo, and can be related to structural problems in the uterus or cervix, or, to systemic problems in the woman herself. But there are many uterine abnormalities that are simply wrinkles that don’t adversely affect your ability to carry a child. Some systemic abnormalities are a potential cause of miscarriage, but they can usually be treated, and most don’t even require treatment. The majority of miscarriages are caused by chromosomal abnormalities, and the risk of miscarriage in such cases can be lessened by performing a genetic analysis of the embryos (using FISH) prior to IVF transfer.

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